The technology field of the present invention relates to an image forming apparatus.
There has heretofore been known an image forming apparatus, which transfers a toner image on a photoreceptor onto an intermediate transfer body (an intermediate transfer belt), and then transfers the toner image, which have been transferred onto the intermediate transfer belt, onto a sheet of paper via a transfer section of a roller method (a secondary transfer section).
The transfer section of such a roller method had a transfer roller and a backup roller which faces the transfer roller, and had a problem that a speed of the intermediate transfer belt changes due to a shock at a time when a sheet of paper enters a nip portion between the transfer roller and the backup roller, resulting in unevenness of a printed image.
In order to prevent such unevenness, there has heretofore been known an apparatus in which a tension roller is arranged downstream of a primary transfer section which transfers a toner image on a photoreceptor onto an intermediate transfer belt and upstream of a secondary transfer section, thereby the speed fluctuation of the intermediate transfer belt is absorbed (refer, for example, to Japanese Patent Application Publication (hereinafter referred to as JP-A) No. H11-268595).
In addition, to prevent the similar unevenness, there has heretofore been known an apparatus in which a flywheel is arranged on an axis of a backup roller facing to a transfer roller of a secondary transfer section, thereby the speed fluctuation of the intermediate transfer belt is absorbed by the aforesaid flywheel to prevent the occurrence of the unevenness (refer, for example, to JP-A No. 2007-264292).
The conventional secondary transfer section and its problems will be described below to facilitate understanding of the present invention.
FIG. 3 is a schematic drawing of the conventional secondary transfer section.
The conventional secondary transfer section 6′ is composed of a backup roller 44′, a transfer roller 61′ facing the backup roller 44′ and is capable of nipping an intermediate transfer belt 4′ and a sheet of paper P, a transfer power source 62′, and a solenoid 63′ which urges the transfer roller 61′ toward the backup roller 44′.
While the sheet of paper P is passing through a nip portion n′ between the backup roller 44′ and the transfer roller 61′, switches of the transfer power source 62′ and an urging device 63′ become ON to transfer a toner image supported on the intermediate transfer belt 4′ onto the sheet of paper P.
The backup roller 44′ is composed such that a solid rubber 443′ is formed around a shaft 441′, and fluorine resin 444′, for example tetrafluoroethylene resin (PTFE poly-tetrafluoroethylene), is coated on the surface of the solid rubber 443′. Since the backup roller 44′ is composed of the solid rubber 443′ (for example, the Asker C hardness of about 65 degrees), the constitution of the backup roller 44′ exhibits a less elasticity.
When a sheet of paper enters the nip portion n′ at the transfer process, the leading edge P1 of the sheet of paper pushes away the backup roller 44′ and the transfer roller 61′ to enter the nip portion n′, and when the sheet of paper leaves the nip portion n′, the backup roller 44′ and the transfer roller 61′, which had been pushed away, come back to the original positions.
Due to the above reason, in the case where a sheet of paper is particularly thick (for example, a basis weight of about 300 g/m2 or more) and the elasticity of the backup roller 44′ is low as was described above, a shock is generated at a time when the sheet of paper enters and leaves the nip portion to cause a large fluctuation of a circling rate of the intermediate transfer belt 4′.
This generation of the shock is similar to a phenomenon which happens when we get on and get off a bump with a bicycle, if air pressure of the tire is high, we feel a shock, and if air pressure is low, we do not feel the shock.
The large fluctuation of a circling rate of the intermediate transfer belt 4′ affects also to the upstream side (for example, a primary transfer section 5′), and causes difference in speed between a photoreceptor 1′ rotating at a nearly constant rate and the intermediate transfer belt 4′. As a result, the large fluctuation causes a problem of generating unevenness of a printed image.
Hereinafter, the unevenness of a printed image, caused by difference of speed between a photoreceptor and an intermediate transfer belt at a time when the sheet of paper enters and leaves the nip portion, is referred to as a shock jitter.
However, the secondary transfer section described in JP-A No. H11-268595 had a problem such that unevenness of printed image occurred due to a shock at a time when a sheet entered the above nip portion (the shock jitter). Therefore, a tension roller had to be arranged to absorb the shock to prevent the occurrence of the aforesaid unevenness, which then led to a larger and more complicated apparatus, resulting in a problem of increase in failure frequency and in a manufacturing cost due to an increase in number of parts.
Further, the secondary transfer section described in JP-A No. 2007-264292 had a problem such that unevenness of printed image occurred due to a shock at a time when a sheet entered the above nip portion (the shock jitter). Therefore, a flywheel had to be arranged to absorb the shock to prevent the occurrence of the aforesaid unevenness, which then led to a larger and more complicated apparatus, resulting in a problem of increase in failure frequency and in a manufacturing cost due to an increase in number of parts.
In view of the foregoing problems, an object of the present invention is to provide an image forming apparatus which is small, relatively trouble-free and low manufacturing cost, and is capable of outputting a high-quality printed image.